This invention relates to semiconductor light emitting devices and manufacturing methods therefor, and more particularly to packaging and packaging methods for semiconductor light emitting devices.
Semiconductor light emitting devices, such as Light Emitting Diodes (LEDs) or laser diodes, are widely used for many applications. As is well known to those having skill in the art, a semiconductor light emitting device includes one or more semiconductor layers that are configured to emit coherent and/or incoherent light upon energization thereof. It is also known that the semiconductor light emitting device generally is packaged to provide external electrical connections, heat sinking, lenses or waveguides, environmental protection and/or other functions.
For example, it is known to provide a two-piece package for a semiconductor light emitting device, wherein the semiconductor light emitting device is mounted on a substrate that comprises alumina, aluminum nitride and/or other materials, which include electrical traces thereon, to provide external connections for the semiconductor light emitting device. A second substrate, which may comprise silver plated copper, is mounted on the first substrate, for example, using glue, surrounding the semiconductor light emitting device. A lens may be placed on the second substrate over the semiconductor light emitting device. Light emitting diodes with two-piece packages as described above are described in Application Serial No. US 2004/0041222 A1 to Loh, entitled Power Surface Mount Light Emitting Die Package, published Mar. 4, 2004, assigned to the assignee of the present invention, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
With multipart mounting packages for semiconductor light emitting devices, the different parts are typically made of different materials. As a result, the thermal impedance for such packages may be higher and a thermal mismatch may result among the various components within a package that may cause reliability problems with a package. For example, problems may result at an interface between a copper metal of a heat sink or cavity with a plastic of a body in which such a heat sink or cavity is mounted. In addition, assembly may be more complicated because of increased piece part counts for the package. In addition, where a sheet metal optical cavity is utilized, a cavity typically can only be fabricated in a limited range of depth and shape configurations. Such multipart packages may also have a larger optical cavity space, resulting in greater volumes of encapsulant being used, which may increase problems related to delamination and/or formation of bubbles within the encapsulant during temperature cycles.
For known power light emitting device packages, a relatively small heat-slug is typically utilized, which may be portion of the associated electrical leads or an isolated piece. A molded plastic body typically occupies a major part of the package and wraps around the heat spreading component. A lens may be mounted on top of the plastic body and surrounding an optical cavity and an encapsulant material may be used to fill the cavity, which may be hard epoxy or may be soft gel. An example of a prior art power light emitting device package is available from Lumileds Lighting, LLC of San Jose, Calif. as described in U.S. Pat. No. 6,274,924. As described in the '924 patent, a molded plastic body, which occupies a major bulk of the package, wraps around a relatively small, isolated heat-slug on which an LED chip or chips may be mounted. A plastic lens may be locked onto the plastic body, for example, using an adhesive, to form a closed space in the heat-slug and a portion of the plastic body. The enclosed space may be filled with a soft gel that may move in and out of the enclosed space through vents with temperature change. Such a product may not be surface mount technology (SMT) compatible, as its packaging materials generally cannot withstand solder reflow temperatures. In addition, during use, the soft encapsulant may tend to form bubbles or delaminate from the enclosure walls, which may adversely affect device reliability. It is also known to form the lens using a dispensing method capitalizing on the viscosity of a resin used in forming the lens.
The use of a pre-molded lens attached by adhesive may encounter some problems in robustness and reliability of the finished product. For example, the manufacturing process for such devices may be inherently inconsistent and the resultant package may be less robust and/or reliable.
It is also known to use transfer molding of epoxy to encapsulate certain low power LED packages, such as miniature surface mountable devices available from from Hewlett Packard Corporation. The epoxy on such devices may provide the structural strength to the package as well as encapsulating the devices inside. However, epoxy tends to be degraded by the electromagnetic energy of blue light, generally generated by some semiconductor light emitting devices, and may become less transmissive to light as a result. The resulting package may, therefore, become dimmer over a relatively short period of time. As such, epoxy may be a less attractive option for encapsulating devices that emit blue light. In addition, epoxy generally has a Coefficient of Thermal Expansion (CTE) mismatch problem with silicone soft gel, which may be used to junction coat the LED chips and their bond wires as the first layer of encapsulant.
It is also known to use casting to encapsulate LED devices with epoxy. This process typically can only be applied to an open chamber, where curing may occur with the epoxy contained in a cup and a lead frame may be inserted inside the cup and be casted when the epoxy is cured. During curing, a level of liquid epoxy is generally free to adjust itself as a result of chemical reactions and shrinkage in volume.